This invention relates to the field of measurement while drilling (MWD) and borehole telemetry. More particularly, this invention relates to measurement downhole of the speed of rotation of a rotating unit, such as a turbine, and transmitting the measured speed parameter to the surface.
In the field of borehole drilling, particularly oil and gas well drilling, the usefulness of systems capable of detecting parameters at the bottom of the drill string and transmitting such data to the surface during the course of drilling has long been recognized. Mud pulse telemetry systems are known and in commercial use for measuring while drilling. Such systems are shown, for example, in U.S. Pat. Nos. 3,982,431; 4,013,945 and 4,021,774, all of which are assigned to the assignee hereof.
One method of well drilling involves driving a drill bit with a downhole turbine which is powered by the drilling mud which circulates in the drill string and in the anulus between the drill string and the wall of the well. These drilling systems are very efficient and are highly desirable for use in straight drilling. However, in order to be effective, the turbine in these systems must be operated within a narrow rotational speed range for optimum power output. Thus, it is important to be able to regulate the speed of the turbine; but regulation cannot be accomplished without knowing the actual speed at which the drill is rotating.
This requirement for and regulating the speed of a drill is, of course, only one example of situations in which it may be desirable to know the speed of rotation of a turbine or other downhole equipment.